The second step when entering Zen meditation is to regulate the breathing. In doing so there are four ways to breathe, i.e., the breath sounds like wind blowing, sighing, Qi flowing, and quiet breathing. The first three are not appropriate for adjusting your breath, while the last one does help regulate it.
Why is the first way called "windy?" Because the breath as it enters and leaves your nose makes a sound like wind blowing. When is breathing called "sighing?" When exhaling and inhaling are uneven in length, even if there is no sound, this condition is called sighing.
What is "Qi flowing" breath? When, while sitting, your breathing, though it makes no sound and is even, is still rough and energy filled, that is called "Qi breathing."
Quiet Zen breathing occurs when there is no sound, not broken, not coarse. Exhaling and inhaling are refined (continuous like a fine thread spinning), aware yet unaware of the breathing process. Body at peace, the feelings become quiet and contented. This condition is called quiet Zen breathing.
Windy breath scatters, sighing binds, Qi breath tires, quiet breath settles you into the Zen meditative state. If when sitting you breathe in the windy or any of the first three "non-regulated" ways, and try to meditate, your heart will be troubled and your mind hard to focus.
If you wish to regulate the heart-mind, there are three techniques that you must rely on. The first is to focus your attention on the lower part of the abdomen three inches or so below the navel, two or three inches within, the center of gravity of the body.
Second, you must relax the body entirely. Third, you must visualize your breath as passing through every pore of your body, going out and entering every joint and crevice, so that there are no obstacles to its even flow.
If you purify the heart-mind and make the breath quiet and hardly perceptible, then when the breathing is thus regulated, worries and anxieties will not arise, and the mind will be easily focused and settled.
The above defines what the practitioner does when first entering Zen meditation, i.e., the means to regulate breathing. To sum it up, essentially to regulate breathing don't breathe too tensely or too loosely.
The third step to take when entering Zen meditation is to regulate the mind. There are two things to remember here. The first is to quell all random images, not letting them pass through your mind freely, and not allowing them room to float or sink in. The second is to keep the heart-mind from being too deep, too shallow, too lax or tense a condition.
What is meant by too deep a condition? If when you are sitting the mind falls into a twilight or dark state and begins to lose its awareness, the head nods and sinks forward, this is called too deep a condition. At this point visualize an imaginary object at the tip of the nose, and bring the mind back to become aware of it, not letting yourself be distracted by random images. This will control the mind from sinking too deep.
What is meant by the shallow or floating condition? If while you are sitting in Zen meditation the heart-mind likes to move about freely, and the body becomes restless, and you visualize and recall unusual external objects, this is called the shallow or floating state.
At this time it is easy to quell the heart-mind by focusing attention down into the lower abdomen, and make it an object of vision. You thus control all random thoughts, keeping your mind settled in Zen concentration. In this way your heart is serene and tranquil.
You ask me "In such a condition isn't it easy for the mind to become too relaxed or too tense?"
My answer is that yes, sometimes indeed this does occur. When you try to concentrate too quickly, fervidly using the mind while sitting, focusing attention toward some conceptual image, you thereby cause the thoughts to enter into your mind's concentrated gate, thus forcing Qi energy upwards, causing the chest and mind to ache.
When this happens, loosen and free the heart's attention, visualizing the Qi energy to flow downward to the abdomen, and thereupon the tension will subside of itself.
If the heart is in a slack state, you will percieve that the attention wanders and the will power fades away, the body's strength ebbs, the mouth salivates, and sometimes darkness will overpower the mind. In such a case you must get hold of your body, control your thoughts, bring the heart-mind back to the state of awareness by focusing on the body, to master yourself.
You must use these means to control slackness. Thus one can know the rightness or slackness of the heart-mind by the foregoing analysis, which shows the way to regulate the heart when beginning Zen meditation
In summary, entering Zen concentration is basically passing from the coarse to the refined. The body is coarse, breathing in between, and the heart is most refined. By regulating the coarse so that it is refined, the heart becomes peaceful. These, then are the Upaya or skillful means to begin Zen meditation.